1. Field of the Invention
The present invention relates to a method of preventing the transfer of an adhesive substance to a ring frame for use in the wafer dicing process, a pressure-sensitive adhesive sheet used in the above method and a wafer working sheet having the above pressure-sensitive adhesive sheet.
2. Description of the Prior Art
The process for producing a semiconductor device includes a dicing step in which a semiconductor wafer having circuits formed through necessary pretreatments is cut and separated into a plurality of chips. In this step, a dicing sheet for fixing a wafer is stuck to a circular or square frame known as a ring frame, a semiconductor wafer is stuck on this dicing sheet and dicing is carried out every circuit to thereby obtain semiconductor chips.
The dicing step is followed by bonding machine expanding and chip mounting steps and further by wire bonding and molding steps to thereby complete production of a semiconductor device. Although the dicing sheet is fixed on the back of the wafer (chip) from the sticking of the wafer to a chip pickup step, after the pickup step, the dicing sheet is manually peeled from the ring frame and the ring frame is cleaned according to necessity and recycled.
In the conventional dicing sheet, a pressure-sensitive adhesive with low adhesive strength has been employed to facilitate the pickup. Accordingly, manual peeling of the dicing sheet from the ring frame has been facile.
However, the conventional dicing sheet having the pressure-sensitive adhesive with low adhesive strength has been unable to indicate a desired performance when a single type thereof is employed despite the need for a tape having higher dicing performance brought about in accordance with the recent-year trend for increase of semiconductor element integration and chip enlargement. Thus, a dicing sheet has been developed which can hold the wafer (chip) at a high adhesive strength during the dicing step but has its adhesive strength reduced by, for example, ultraviolet irradiation in the pickup step to thereby enable easy pickup of chips (see, for example, Japanese Patent Laid-open Publication Nos. 60(1985)-196956 and 61(1986)-28572).
Further, when a chip is die bonded, a die bonding adhesive such as one composed of an epoxy resin has been applied to the back of the chip. This operation is extremely time-consuming. Consequently, in recent years, a dicing sheet has been developed which has an adhesive layer capable of simultaneously serving as a wafer fixing pressure-sensitive adhesive and as a die bonding adhesive (see, for example, Japanese Patent Laid-open Publication Nos. 60(1985)-35531 and 2(1990)-32181).
In the former ultraviolet-curable dicing sheet, the adhesive is less irradiated with ultraviolet at the part where the dicing sheet is stuck to the ring frame (overlap width part) although the lowering of the adhesive strength is marked at the part irradiated with ultraviolet. Thus, peeling the dicing sheet from the ring frame is difficult because at the overlap width part the adhesive maintains a high adhesive strength and also its cohesive strength is small. Even if the peeling is effected, the adhesive remains on the ring frame (adhesive transfer).
In the latter dicing sheet, the adhesive strength at the interface between a base sheet and the adhesive layer is often set at low for transferring the adhesive layer to the chip at the time of chip mounting. Thus, peeling of the dicing sheet from the ring frame has often been accompanied by peeling at the interface between the base sheet and the adhesive layer of the dicing sheet to result in removal of only the base sheet while the adhesive is left on the ring frame.
This has unfavorably increased the frequency of cleaning for removing the adhesive from the ring frame and also shortened the life of the ring frame.